The development of VLSI semiconductor devices of the dynamic random access memory (DRAM) type is well known. Over the years, the industry has steadily progressed from the DRAMS of the 16K type (as shown in U.S. Pat. No. 4,181,701) to DRAMS of the 64K type (as shown in U.S. Pat. No. 4,055,444) to DRAMS of the 1MB type as shown in U.S. Pat. No. 4,658,377 and progressed to DRAMS of the 4MB type. The 16MB DRAM, wherein more that 16 million memory cells are contained on a single semiconductor chip is another generation of DRAMS to be produced.
In designing VLSI semiconductor memory devices of the 16MB DRAM type, designers are faced with numerous challenges. One area of concern is power consumption. The device must be able to power the increased memory cells and the supporting circuits. However, for commercial viability, the device must not use excessive power. The power supplies used and the burn in voltage for the device must also be compatible with the thin gate oxides in the device.
Another area of concern is the elimination of defects. The development of larger DRAMS has been fostered by the reduction in memory cell geometries such as illustrated by U.S. Pat. No. 4,240,092 and U.S. Pat. No. 4,721,987. The extremely small geometries employed by the 16MB DRAM is manufactured using sub-micron technology. The reduction in feature size has meant that particles that previously did not cause problem in the fabrication process, now can cause circuit defects and device failures.
In order to ameliorate defects, redundancy schemes have been introduced. The redundancy schemes normally consist of a few extra rows and columns of memory cells that are placed within the memory array to replace defective rows and columns of memory cells. Designers require new and improved redundancy scheme in order to effectively and efficiently repair defects and thereby increase the yields of the 16MB DRAM chips.
Another area of concern is testing. The device must have circuits to allow for industry standard X16 parallel tests. In addition, other circuits and test schemes are needed for internal production use to verify operability and reliability.
The options that device should have is another cause for concern. For instance, some customers require a X1 device, while others require a X4 device. Some require an enhanced page mode of operation.
Another cause for concern is the physical layout of the chip. The memory cells and supporting circuits must fit on a semiconductor chip of reasonable size. The size of the package device must be acceptable to buyers.
New design strategies and circuits are required to meet the above concerns, and other concerns relating to the development of the dynamic random access memory devices.
FIG. 1 illustrates a multiplier circuit 200. The multiplier circuit includes two stages, namely a comparator stage 202 and multiplier stage 204. FIG. 1 illustrates a multiplier circuit to achieve an output voltage represented by V.sub.OUT of 3.3 volts. Since the reference voltage, V.sub.ref, is approximately 1.25 volts, the output voltage V.sub.OUT, across resistors 208 and 210, is 3.3 volts. As the ratio of V.sub.OUT /V.sub.ref is not an integer, this ratio may be produced by a voltage divider through a resistor string. However, one problem with such a multiplier used with DRAMS is that these resistors of the multiplier are constantly conducting and the multiplier is constantly using current. One object of designers of the DRAM voltage multiplier is to minimize this constant current in order to save power. One method of reducing this stand-by current is to increase the resistance of resistors 208 and 210 by increasing the layout area, resulting in a resistor that is long and narrow. However, a disadvantage of a long and narrow resistor is the associated parasitic capacitance. The parasitic capacitance, illustrated in FIG. 1 by capacitors 206 and 212 affect the feedback circuit of FIG. 1 adversely by introducing additional phase shifts resulting in circuit instability. If the feedback circuit is unstable, any small noise which maybe introduced is amplified, and the feedback circuit tends to oscillate by the amplification of this noise.